Compact high-brightness mid-IR light sources have many applications in medicine, spectroscopy, ranging, sensing and metrology. For mass market applications such sources need to be highly robust, have long term stability and also include a reduced component count with a high degree of optical integration.
Mid-IR and far-IR wavelength ranges, as used in the present application, includes wavelengths from about 2-10 μm and 5-100 μm, respectively. Prior art mid-IR to far-IR light sources are limited in output power, spectral coverage for wavelengths>5 μm, wavelength tunability, and/or limited optical conversion efficiency to the mid-IR.
Difference frequency generation (DFG) may be utilized for generation of mid-IR or far IR outputs. Performance constraints can arise as a result of the limited coherence of the DFG pump sources and/or relatively complex system assembly involving typically two separate optical paths, a path delay adjustment stage, as well as a beam-splitter.
Frequency down-conversion via DFG using systems based on mode locked fiber lasers has recently attracted a lot of attention, see for example D. G. Winters et al., Subpicosecond fiber-based solution-tuned mid-infrared source in the 9.7-14.9 μm wavelength region, Opt. Lett., vol. 35, pp. 2179-2181 (2010) and T. W. Neely et al., ‘High-power broadband laser source tunable from 3.0-4.4 μm based on a femtosecond Yb:fiber oscillator.”, Optics Letters, vol. 36, pp. 4020-2022 (2001) as well as G. Krauss et al., All-passive phase locking of a compact Er:fiber laser system, Opt. Lett., vol. 36, pp. 540-542 (201)).
The main drawbacks of the prior art systems were, for example, limited coherence of the DFG output (Winters and Neely) or the lack of frequency down-conversion (Krauss). In addition these systems were relatively large and incorporated many bulk optical components.
A need exists for compact, wavelength-tunable and robust high power coherent DFG sources with reduced component count and broadband IR capability.